The present invention relates generally to signal processing techniques, and more particularly to filtering techniques for use in echo cancellation applications.
An echo canceler is a type of filter utilized to reduce or eliminate the presence of signal xe2x80x9cechosxe2x80x9d or reflections, e.g., in transmission lines of telecommunication systems. For example, echoes characterized by a partial reflection of an incoming signal are produced in transmission lines of a telecommunication system whenever an impedance discontinuity or mismatch exists, such as at the junction between a four-wire transmission path and a two-wire transmission path in a telephone network or other telecommunication network. If the so-called echo-path delay is very small, then the reflected signal is typically not noticed at the source of the original signal. However, if the echo-path delay is otherwise, then the echo becomes, qualitatively, annoying to the source, i.e., the talking party. A conventional echo canceler synthesizes an estimate of the echo signal and subtracts the estimate from an outgoing signal to effectively xe2x80x9ccancel outxe2x80x9d the echo signal. Such echo cancelers are described in greater detail in, e.g., D. L. Duttweiler, xe2x80x9cA Twelve-Channel Digital Echo Canceler,xe2x80x9d IEEE Transactions on Communications, May 1978, pp. 647-653.
In telecommunication network applications, echo cancelers are often installed as stand-alone pieces of equipment with digital pulse-coded modulation (PCM) interfaces. The echo paths these echo cancelers see therefore necessarily contain the xcexc-law or A-law quantization generally associated with the use of PCM. This quantization limits the maximum attainable echo-return-loss enhancement (ERLE) to about 35 dB. However, very large scale integration (VLSI) technology has now progressed to the point where it has become feasible to integrate an echo canceler on a codec chip. If the codec is an oversampled codec, as is typically the case, then it usually includes internal linear PCM encodings of transmit and receive signals. These digital representations generally exhibit signal-to-noise ratios (SNRs) greater than the 35 dB ERLE limit associated with PCM xcexc-law quantization, at least for larger amplitude signals. Therefore, by moving the echo canceler inside the linear-to-companded transcoders, it appears that it would be possible to provide SNRs greater than the above-noted 35 dB limit.
Unfortunately, another problem exists just below the 35 dB limit imposed by xcexc-law quantization. More specifically, echo paths generally contain low-frequency poles introduced by highpass filtering, equalization, balance networks, and/or terminating loops. These low-frequency poles introduce long, low-frequency tails into the echo path impulse response the canceler is to synthesize. A conventional canceler can only truncate these tails. The amplitude of these low-frequency tails is generally weak enough that the error introduced by such truncation is comparatively small and less than the above-noted 35 dB limit imposed by xcexc-law quantization. Nonetheless, in applications in which it desirable to achieve an ERLE of more than 35 dB, and especially if the canceler is short, i.e., contains fewer than about 64 taps, truncation becomes problematic.
A need therefore exists for improved echo cancellation techniques which provide substantially improved performance relative to the above-described conventional truncation approach.
The present invention provides an improved echo canceler including an adaptive filter having as its basis functions a set of tapped delay line basis functions and one or more additional basis functions implementing quadrature responses of a complex pole. The frequency of the complex pole is substantially equal to the frequency of a dominant pole of an echo path associated with the echo canceler. The dominant pole of the echo path may be forced to a known location by introducing a highpass filter into the echo path.
In an illustrative embodiment of the invention, an echo canceler includes at least one pair of filter taps which correspond to a fixed complex pole matched to a low-frequency complex pole of a highpass filter arranged in an echo path of the echo canceler. The highpass filter may be an nth-order Butterworth, Chebyshev or elliptic highpass filter. The echo canceler basis functions thus formed include the usual flat-delay basis functions of a conventional echo canceler, but augmented by a pair of infinite-impulse-response (IIR) filters with components in quadrature. The dominant complex pole of the highpass filter is the complex pole that is closest to the unit circle. It substantially controls the tail of the impulse response of the highpass filter. The frequency of the complex pole of the pair of filter taps in the echo canceler is selected as substantially equivalent to the frequency of this dominant complex pole of the highpass filter. This echo canceler arrangement results in a substantial improvement in attainable echo-return-loss enhancement (ERLE).
The techniques of the present invention may be implemented in a straightforward manner in a digital signal processor (DSP) or other type of signal processing device. Advantageously, the invention achieves substantial improvements in terms of ERLE relative to conventional tail truncation approaches. For example, an illustrative embodiment of the invention can provide an ERLE on the order of 76 dB. These and other features and advantages of the present invention will become more apparent from the accompanying drawings and the following detailed description.